This invention relates to semiconductor memory devices and in particular to a dynamic memory device which substantially prevents soft errors caused by alpha particle radiation.
As the semiconductor industry is continuing the trend toward higher levels of integration in memory circuits the presence of soft errors have become prevalent due to the passage of heavily-ionized radiation through the memory array area and resulting in the loss of stored data. When an alpha particle penetrates the surface of the memory substrate, it can create enough electron hole pairs near a storage node to cause a random, single bit error (i.e., soft error). These errors are caused by a substantial loss of stored charge in the memory storage capacitor of the semiconductor device. At smaller levels of integration (.e.g., 16K bit memories and smaller) the effect of alpha particle radiation on the stored charge is negligible since each capacitor maintains a relatively large capacitance and, thereby, a large charge. At higher levels of integration, however, the area of the storage capacitor is smaller which thereby reduces its capacitance; thus, the effect of radiation can appreciable effect the smaller stored charge and result in a change in the binary state of the memory.
This problem is particularly acute in dynamic memories since each memory circuit comprises two elements including a single switching transistor and a charge storage capacitor. As a result, there is no means available to draw off the excess charge created by the radiation. Generally, static memories, on the other hand, incorporate six elements including two switching transistors having opposite states. Consequently, the additional charge created by the radiation may be drained off by the on transistor. At higher integration levels, however, static memory can also be adversely effected by radiation due to its higher density of components.
The prior art has attempted to mitigate soft error problems by various techniques. One technique increases the storage density of the charge storage memory capacitor by either making the gate oxide thinner or changing the composition of gate oxide to increase its dielectric constant. Making the gate oxide thinner, however, increases the electric field thereby producing breakdown and associated reliability problems. Moreover, changing the composition of the gate oxide is likewise ineffective since other compositions are not as reliable as SiO.sub.2 and, at best, can only produce a small change in storage density. In any event, the purity of other compositions are still unknown and require further investigation and development.
Another prior art technique utilizes a sensor amplifier coupled to the memory device and having improved sensitivity. By increasing the signal-to-noise ratio of the amplifier it can detect signal changes produced by the radiation. Such an amplifier, however, is impractical to construct since the S/N ratio is limited as the integration level increases. Furthermore, at larger integration levels the noise level increases.
A further prior art technique utilizes chip coating. With this method, the memory chip is coated with a contamination-free organic material such as a polyimide resin. This technique, however, does not prevent radiation which is generated by the chip composition itself and impurities within the chip package. Moreover, the process of chip coating is costly and time consuming. While the industry has attempted to improve the purity of the chip itself and its packaging these attempts have been impractical.
The problems of the prior art memory devices discussed above will be explained with reference to FIGS. 1-3.
Conventionally, dynamic type memory devices have been designed by arranging unit memory cells in the form of a matrix as shown in FIG. 1. That is, a switching MOS transistor Q.sub.1 and a capacitor C are connected in series between a bit line BL and a power supply P.sub.O ; the gate electrode of transistor Q.sub.1 is connected to a word line WL. The particular switching transistor of the unit memory cell which is selected by the word and bit line turns ON, so that a charge is stored in the capacitor which performs the memory operation. The stored charge is subsequently read out on the bit line via the switching transistor.
FIG. 2 shows a plane view of the pattern layout of the circuit of FIG. 1, wherein one unit cell is surrounded by a dot-dashed line 11. In the drawing WL.sub.1 and WL.sub.2 are word lines formed by a low resistance wiring layer. BL.sub.1 and BL.sub.2 are bit lines formed by, for example, an aluminum wiring layer, and C.sub.G is a low resistance layer for forming an opposite electrode of the capacitor. Switching MOS transistor and MOS capacitor are represented by broken lines 12 and 13, respectively.
In an n-channel dynamic memory device formed on p-type semiconductor substrate, it is important to determine whether many electrons are present at a memory node N or whether a few electrons are present at node N; the first condition corresponds to a "0" (low level) and the second condition corresponds to a "1" (high level). The evaluation of the stored information ("1", "0") is accomplished by comparing the charge read out on the bit line with a reference charge of a dummy cell by utilizing a sensor amplifier. Usually, the memory node N comprises a n-type diffused region wherein an opposite electrode P is connected to a power supply potential and the MOS capacitor is formed between the opposite electrode P and the node N.
Since the dynamic memory device described above stores information by storing a charge in the capacitor, an erroneous reading can occur if the stored charge is lost by undesired leakage. Particularly, as discussed above, such erroneous readings are termed soft errors and are caused by alpha-radiation. In the circuit configuration shown in FIGS. 1 and 2, the quantity of the charge stored in the capacitor is decreased as the circuit is constructed at higher integration densities. Therefore, the memory content is easily destroyed when alpha-particles impinge the semiconductor substrate. As shown in FIG. 3, the alpha-rays applied to the semiconductor substrate 14 through its package generate a large number of electron-hole pairs along the trajectory of the radiation. As a result, the total quantity of the change caused by the radiation is in the order of 100 fC (femto-coulomb=10.sup.-15 C). Within the generated carriers, electrons with high mobility are likely to be collected if an n-type region 15 formed in substrate 14 is positioned along the trajectory of the radiation.
On the one hand, it is known that the carriers generated by alpha-rays are collected during an extremely short time period. When alpha-rays .alpha. impinge an n-type region 15, formed in a p-type substrate 14, the current waveform of the generated electrons flowing into region 15 is represented by a pulse current having a narrow pulse width as shown in FIG. 4. The continuous time t.sub.d (i.e., 0.2 to 0.3 n sec) of the pulse current is sufficiently short as compared with the internal node time constant of the LSI. During this time, since a large number of electron-hole pairs are produced along the trajectory of the radiation, the electrical conductivity increases near the trajectory and the carriers are collected at the n-type region 15 along the trajectory. This is called funnelling effect. Since the electron-hole pairs are freely diffused into the semiconductor substrate 14, the high conductivity zone disappears after the time t.sub.d (hereinafter referred to as the funnelling time constant). Therefore, it is difficult to collect the carriers.
With respect to the memory cell, the memory node N is a n-type region, and if a "1" is stored, the electrons flow easily into the n-type region because the potential is low. Consequently, a malfunction occurs whereby a stored "1" becomes a "0". In conventional devices, the memory node capacitance is in the order of 40 fF, and the quantity of charge to be stored is 200 fC (40 fF.times.5 V) assuming the power supply voltage is 5 V. Since the total quantity of charge generated by the incident radiation is in the order of 100 fC, in the generated charge reaches 1/2 of the memory node capacitance. Accordingly, the likelihood that the memory content will be destroyed by carriers due to alpha-radiation is increased. In order to reduce the soft error rate, it has become necessary, for example, to increase the memory node capacitance to a sufficiently large value to neglect the influence of carriers caused by alpha-rays. However, as discussed above, increasing the capacitance produces serious problems if high integration density is desired.